Section 255(3.1) describes two distinct but related offences. The offence of "impaired driving" prohibits the operation or care and control of a vehicle while the person's ability to operate it is impaired by drug or alcohol. The offence of "over 80" prohibits the operation or care and control of a vehicle while the person's blood alcohol concentration (BAC) is over 80mg per 100ml of blood.

These offences are some of the most heavily litigated of all criminal offences. The proof of impaired driving has basis largely on the observational evidence of eye-witnesses, usually the investigating officer. They can become complicated when involving drugs rather than alcohol as it requires a Drug Recognition Expert. The proof of the Over 80 offence requires a procedure of taking a breath or blood sample and then extrapolating the estimated BAC at the time of the offence. There are several short cuts to proof that are available under s. 258.

Both offences have the common element of operation or care and control, which becomes most uncertain when examining whether there was care or control arising from the risk of the vehicle being put in motion. These offences also often engage Charter rights under s. 8 and 9, 10(b).

Operation while impaired
253. (1) Every one commits an offence who operates a motor vehicle or vessel or operates or assists in the operation of an aircraft or of railway equipment or has the care or control of a motor vehicle, vessel, aircraft or railway equipment, whether it is in motion or not,

(a) while the person’s ability to operate the vehicle, vessel, aircraft or railway equipment is impaired by alcohol or a drug; or

(b) having consumed alcohol in such a quantity that the concentration in the person’s blood exceeds eighty milligrams of alcohol in one hundred millilitres of blood.

The mens rea of the offence of impaired driving is made out by the accused's voluntary consumption of alcohol for the purpose of becoming intoxication or the accused "acting recklessly, aware that impairment could result, but persisting despite the risk".
[2]

It is not necessary that the accused have actual knowledge of the effects of drugs or alcohol. Proof of recklessness is sufficient.[3]

Where the voluntary consumption of alcohol is proven, there is a rebuttable presumption the mens rea is made out.
[4]

The intent to be impaired can be negated in certain circumstances where the accused's drink may have been drugged.[5]

Under s. 2, "motor vehicle" is defined as "a vehicle that is drawn, propelled or driven by any means other than muscular power, but does not include railway equipment;" Certain vehicles, such as scooters, are powered by either pedal or motor, the crown will generally have to prove that the vehicle was being operated by motor power at the time of the offence.[1]

A vehicle that is inoperable, such as were it is out of gas, will still be a motor vehicle.[2]

Kienapple does not prevent a conviction for dangerous driving causing death or bodily harm and impaired driving causing bodily harm or death. The first offence concerns the ability to operate a vehicle while the second offence focuses on the manner in which the vehicle was operated.[1]